Right now the sites are completely different entities... this requires seperate logins for the time being. If you're using the autologin and save your profile for both sites, then it's simply done behind the scenes.
One of the coming changes will be the removal of 75%+ of the need for cookies and client side session capturing. This means that the database will track that you've logged in and not require you to login at any other site.
No estimate on when this project will begin or and Estimated completion date.